Short answers, pulled from the story.
Street Fighter X Tekken is a 2012 crossover fighting video game published by Capcom, co-developed with Dimps. It features characters from both the Street Fighter and Tekken franchises competing in tag-team matches, and was released for PlayStation 3, Xbox 360, PlayStation Vita, and Windows.
Street Fighter X Tekken sold over one million copies within two months of release. By the 31st of March, 2012, it had reached 1.4 million units worldwide, and climbed to 1.7 million by the 31st of March, 2014. Capcom had expected the game to sell 2 million units, leaving it roughly 40% short of their target.
Twelve additional characters released as paid downloadable content were discovered to already exist on the retail game disc. Capcom stated the characters were locked on the disc to reduce the hard drive space required of players. The explanation was widely criticized, and ScrewAttack included it on their 2012 list of the top ten Capcom mistakes.
The PlayStation 3 and Vita versions included five exclusive characters: Cole MacGrath from Infamous, Sony mascots Toro Inoue and Kuro (dressed as Ryu and Kazuya), Pac-Man piloting a Mokujin-style mech, and Mega Man in his North American box art design. Mega Man and Pac-Man became free downloads for PlayStation 3 owners on the 13th of March, 2012.
Pandora Mode activates when one member of a player's team drops below 25% health. That character is sacrificed, granting the surviving fighter increased strength and an infinite Cross Gauge. The mode has a strict time limit, and the player automatically loses the round if the opponent is not defeated before it expires.
The game was announced during the Street Fighter IV panel at the 2010 Comic-Con. Tekken producer Katsuhiro Harada walked out of the crowd, handed out free copies of Tekken 6, and confirmed the crossover alongside Street Fighter producer Yoshinori Ono. The announcement followed teasing at EVO 2010 in Las Vegas, Nevada, where audiences initially suspected a new Darkstalkers game.